This invention relates to molten carbonate fuel cells and, in particular, to cathode side hardware employed in such cells.
As used herein, the term xe2x80x9ccathode side hardwarexe2x80x9d is defined as the current collector and/or the bipolar plate on the cathode side of a fuel cell. In molten carbonate fuel cells, the cathode-side hardware is in direct contact with the porous cathode in which alkali carbonate electrolyte is stored. FIG. 1 schematically shows such a fuel cell.
As shown, the fuel cell 1 comprises a cathode 2 and an anode 3. Between the cathode 2 and the anode 3 is a matrix 4 containing the alkali carbonate electrolyte. Adjacent the anode 3 is a corrugated current collector 3a and a bipolar plate 3b. Adjacent the cathode 2 is the cathode side hardware 5 comprising corrugated current collector 5a and bipolar plate 5b. 
At the operation temperature (approximately 650xc2x0 C.) of the cell 1, the stored electrolyte melts and wets the surface of the cathode-side hardware 5. Moreover, at this temperature, the cathode atmosphere comprises an oxidizing gas mixture including O2, N2, H2O and CO2. Under these conditions, the cathode current collector, which is commonly formed from a low-cost austenitic stainless steel, is subjected to so-called hot corrosion.
More specifically, a multi-layer corrosion product is formed on the current collector surface. This corrosion product includes a porous outer layer which is a Fe-rich oxide which generally converts to LiFeO2. The product also includes an underlying (inner) layer. This inner layer comprises spinel compounds (Fe, Cr)3O4, and is relatively dense. It also provides some corrosion protection for the current collector, i.e., protects the current collector from further attack.
As can be appreciated, the fuel cell 1 experiences significant ohmic voltage loss due to the interfacial resistance (about 60 xcexa9-cm2 at the beginning of life, equivalent to xcx9c2% power loss) between the cathode and the cathode current collector resulting from the formation of this outer surface oxide (electrical resistivity xcx9c300 xcexa9-cm2 for LiFeO2). The interfacial electrical resistance continues to increase as the oxide thickens with time. It has been estimated that an approximately 2.5 xcexcm thick layer of LiFeO2 can cause more than 50 mV ohmic voltage loss at 160 mA/cm2.
The hot corrosion of the current collector also accelerates electrolyte loss. This is due to the formation of the aforementioned Li-containing corrosion products, such as LiFeO2, and K-containing corrosion products, such as K2CrO4, which consume valuable lithium and potassium carbonate electrolyte. Significant electrolyte loss, in turn, can cause electrode performance decay, ionic resistance increase, and reactant cross-over.
Accordingly, it has been recognized that the durability of a carbonate fuel cell can be enhanced by reducing the hot corrosion of the cathode-side hardware and the associated electrolyte loss. More particularly, it has been proposed to provide a protective and conductive oxide formation on the cathode-side hardware to realize low contact resistance and low electrolyte loss (C. Yuh, Proceedings of Molten Carbonate Fuel Cell Technology, Vol. 90-16, pp. 368-377 (1990)).
Specifically, efforts have been made in this regard in the prior art to identify a metallic alloy that has excellent corrosion resistance in the cathode environment for use in forming the cathode side hardware (U.S. Pat. No. 5,399,438, Japanese Patent Document 8020846, Japanese Patent Document 6305294, U.S. Pat. No. 5,643,690). Also, other efforts have been made to apply corrosion resistant coatings comprised of W, Mo, Ni, Cu, Cr, Co, Ag, or Pa to the hardware. Doping agents, such as cobalt, manganese, copper or magnesium, have additionally been applied to the hardware or used as a component of the steel alloy forming the hardware (German Patent DE 19523635, Japanese Patent Documents 61071559 and 61024156). The purpose of such doping is to change the composition of the corrosion products, lowering their electrical resistivity. In all the aforementioned coating and doping methods described above, a metallic coating is applied to the hardware surface prior to use.
In a number of other types of applications, so-called xe2x80x9csol-gelxe2x80x9d type coatings, i.e., thin dense ceramic coatings, have been developed for corrosion protection (D. W. Richerson, xe2x80x9cModern Ceramic Engineeringxe2x80x9d, Marcel Dekker, 1992). U.S. Pat. No. 5,874,374 describes a method for producing continuous thin film sol-gel type coatings as an electrical component for surface protection against harsh environments. These coatings are used for solid oxide fuel cells and electro-ceramic membranes, porous filters and membranes, and to provide surfaces with desired optical and decorative properties. In the method of this patent, a salt-polymeric aqueous solution is first converted to a metal cation/polymer gel. The metal cation/polymer gel is then further treated to form a structural mass or thin ceramic layer.
Conductive sol-gel LiCoO2 coatings have also been developed for reducing NiO cathode corrosion (S. T. Kuk, et al., Abstracts 1996 Fuel Cell Seminar, pp. 367-370; S. W. Nam, et al., Abstracts 1998 Fuel Cell Seminar, pp. 142-145). In this case, the sol-gel precursor solution contains lithium acetate, potassium acetate, acrylic acid or PVA, and water. With this coating, the corrosion of the NiO cathode was found significantly reduced.
It is an object of the present invention to provide cathode side hardware which has improved performance characteristics;
It is also an object of the present invention to provide cathode side hardware which exhibits reduced susceptibility to hot corrosion;
It is yet a further object of the present invention to provide cathode side hardware which maintains a low electrical resistance with use.
The above and other objects are realized in cathode side hardware by forming the hardware to have a thin film of conductive ceramic coating. Preferably, the coating materials are LiCoO2 or Co doped LiFeO2 and the coating is realized using a sol-gel process.